1. Field of the Invention
The present invention relates generally to short circuit or overload protection circuits and, more particularly, to a circuit of this type which disconnects an electrical load from a circuit and then reconnects the load with this procedure being repeated a predetermined number of times in an attempt to maintain electrical connection between the circuit and the load unless a true short circuit or overload situation exists.
2. Description of the Prior Art
Many different types of sensors are used in a manner in which they connect or disconnect a load from a source of voltage in response to a sensed condition. As an example, a photoelectric detector can be connected in series between a voltage source and an actuator so that the circuit is completed and the actuator is energized or deenergized when the photoelectric sensor detects a predetermined condition. Other types of sensors, such as a proximity detector, can be used in a similar manner.
A serious problem can occur when sensors are used in the manner described above. When the circuitry within the sensor causes the electrical load to be connected in series with the voltage source, the current flowing between the voltage source and the load passes through a portion of the sensor's interface circuitry. If a short circuit condition exists external to the sensor, such as in the external load, excessive currents can flow through portions of the sensor interface circuitry. This excessive current can damage or possibly destroy components within the sensor.
For these reasons, it is well known that an overcurrent detection circuit should be included as part of the sensor circuitry. Many different concepts are known to those skilled in the art for detecting an excessive current and for reacting to that high current. When an excessive current is detected passing through a portion of the sensor circuit, two reactions are generally known to those skilled in the art. First, the overload protection circuitry can immediately disconnect the load from the voltage source and require a manual reset operation before operation of the load can continue. Alternatively, some overload protection circuits temporarily disconnect the load from the voltage source and, after a predetermined period of time, reconnect the load to the source of electrical power. The overload protection circuit disconnects the load from the voltage source a second time. This procedure is continued until the overload condition is corrected or the power is manually turned off. Both alternatives can be disadvantageous, depending on the circumstances of the overcurrent condition. Both of these alternative schemes which are known to those skilled in the art can result in disadvantageous circumstances. For example, if the circuit is designed to immediately disconnect the load from the voltage source upon the first occurrence of an apparent overload condition, the circuit may react to a spurious and essentially harmless short duration current pulse, such as a noise transient, that triggers its operation. If a single overcurrent condition is sufficient to disconnect the load from the source, this type of occurrence would result in the necessity of a manual reset. In many circumstances, the inefficiency and lost time caused by this type of requirement for a manual reset is unacceptable. The alternative arrangement, which reinitiates current following each interruption when the circuit disconnects the load from the voltage source, can create severe damage to the circuit if the cause of the overload condition is a true short circuit of a permanent nature. By continually reinitiating the flow of current through the circuit, the overload protection circuit will place the affected components under repeated stress due to the magnitude of the current and this could eventually result in severe degradation or destruction of the affected circuit components. Therefore, it can be seen that both of the options described above can be disadvantageous in certain circumstances. The first option of immediately disconnecting the load upon the first occurrence of an overcurrent condition is not well suited to circumstances where short duration spurious current pulses can occur and the second option is not well suited in conditions where a true permanent overload condition exists.
U.S. Pat. No. 4,795,980, which issued to Schmitt on Jan. 3, 1989, discloses a device for monitoring short circuits and overloads in electronic proximity switches. The device described in this patent comprises a testing circuit which includes a current pickup which measures the current in the output circuit of a respective proximity switch which controls the output current flow by switching off or limiting the output current flow. The device also includes a circuit which provides a testing cycle for the current measurement with first and second delay times which control the output current during a short circuit or overload and a waiting time, substantially longer than the first delay time, between the start of two successive delay times. The length of the first delay time is chosen with respect to the time constant of the respective transmission line to be connected.
For the reasons described above, it would be significantly beneficial if an overload protection circuit was available which avoided the disadvantage of both of the procedures presently known to those skilled in the art.